(1) Field of the Invention
The present invention relates to a method and an apparatus for adding water to grains such as wheat grains for adapting the grains to subsequent processes, and more particularly to a method and an apparatus for making a precise control of an amount of water addition to the grains.
(2) Description of the Related Art
Conventionally it has been well known to irradiate microwaves to an object and to measure water content values from microwave values obtained from their absorption characteristics based on the magnitude of water contents of the object. Particularly for adding water to wheat grains or tea leaves, use of the water content measurement by means of microwave detection has been known. In such a case, the water contained in the grain itself and the water attached to a peripheral portion of the grain immediately after the water addition are simultaneously measured and, on the consideration that the water on the peripheral portion of the grain is absorbed thereinto with a lapse of time, the water content is calculated by making various corrections in factors such as a temperature, a thickness of each grain and a density of the grains.
That is, at a time immediately after the water addition to the grains, since the water is in a state in which it is attached to a surface of each grain and also since such water attached is necessarily included when the water content of the grain immediately after the water addition is measured, the water content value obtained then is only an estimated water content value on the estimation that the water attached to the periphery of grain will be absorbed into the inside of the grain with a lapse of time. Since the water content value measured at this stage includes an element of estimating the water absorption to the grain after the water addition, it is difficult in practice to carry out the measurement of water content with a high precision. That is, in carrying out such measurement, the measured microwave value is converted into the water content value by using a calibration curve which is determined by water content value obtained in advance and the microwave value. In the case where the water content is thus obtained from the calibration curve, the corrections are made in factors such as a temperature, a grain thickness and a grain density, and the measurement is made for the estimated water content value as a substitute for the absolute water content value. However, the precision of the water content value obtained in this way can be expected to have a precision only in the order of 1%. Depending on purposes or uses, the precision demanded for the water content value may be lower in some exceptional cases, but under the current practice a value of low precision cannot be utilized as reference for control particularly in the case of water addition to wheat grains.
The water addition to the wheat grains is very important for purposes of obtaining wheat flour in its best property and enhancing the yield of the flour. For the flour-milling industry, it is important from the aspect of the weight of the end flour product that, within the limit in which the property of the flour is not caused to be deteriorated, the water content value be controlled to as high as possible, for example, up to about 17%. It is known that, in wheat grains (for example, hard wheat grains), the grinding thereof results in 2% of water loss from the water content value (for example, 16%) of the water added raw grains. The water addition to the raw grains is made with the above matters taken into consideration. However, since the water content of the wheat grains after being ground is proportional to the magnitude of the water content of the water added wheat grains and also since no water can be added to the wheat grains after being ground, it is to be noted that a stable and uniform water adjustment during the water adding stage to the raw wheat grains is very important.
Generally the amount of the grains processed during the milling process is very large so that a difference in the water content even in the order of 1% in weight largely affects the magnitude of the total weight of the end products, which in turn largely affects the sales price of the final end products. Thus, it is necessary that the difference of the water adding amount during the water adding stage be made as small as possible. The conventional water addition control using microwave values is based on the estimated water content value whose precision can be only up to about 1% as already explained, and this is far from meeting the requirements.
In the case of wheat flour, the difference in the water content has a direct bearing and a large influence on the magnitude in weight of the end products so that the error in the order of 1% means that, if the error is assumed to be 1% in weight of the water content and the processing amount is assumed to be 200 tons per day, the difference per day amounts to 2 tons which is 1% of the processing amount of 200 tons per day. Thus, assuming that there are 200 operating days in a year, the total loss per year results in 400 tons. Conversely, if this difference of 1% is controlled advantageously, an annual profit of 400 tons results under the same condition. Naturally, this trend will be more significant with an installation having a capacity of processing a larger amount of grains. Assuming that the price of the wheat flour is JP.Yen.140/Kg., the difference of 1% in weight results either in a loss or a profit of JP.Yen.56,000,000 annually.
Incidentally, for the grains having a variety of water content values, in order to apply an appropriate amount of water thereto in such a way that their water content values become constant, a feedback control method has generally been carried out. In the feedback control method in which the water content is detected by a microwave detection device, the estimated water content value of the water added grains is measured, and then this estimated water content value and the target water content value are compared, whereby the water to be added is feedback-controlled in accordance with the difference resulting from the comparison. The resulting difference may be large because the measurement error in the estimated water content value may eventually be enlarged to as large a value as several percents by calculation, so that the value involving a large error has heretofore been used only as an error admitted yard stick, and in practice the tempering process to follow is relied upon for correcting the error. With respect to wheat flour, etc., it is said that the presence of unevenness in water in the raw wheat grains to be ground is detrimental to the flour milling so that the quality of the end product depends very largely on the water adding step or process.
Further, the fact that the raw grains in the same lot does not necessarily mean that the water content values thereof are the same, so that it is necessary to take the variations therein into account. However, as already explained, since it is not possible in practice to make a precise measurement of the water content values of the grains after the water addition thereto, it is not possible to make an exact judgment as to whether the water content value adjusted in accordance with the water content of the grains before the water addition thereto is accurate or not.
A further method to be considered is one in which the water content of the raw grains is measured by a resistance type water content meter having a comparatively high precision and to control the amount of water content by a feed-forward control method. However, this resistance type is unsuited for the continuous control of a large amount of grains because the measurement intervals are long, the time required for the measurement is long, and the amount of sample is small.